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A complete overhaul. The original implementation in SI-5830 (#821) was pretty buggy.
[from the doc comment of `collapseGuardedCases`:]
Collapse guarded cases that switch on the same constant (the last case may be unguarded).
Cases with patterns A and B switch on the same constant iff for all values x that match A also match B and vice versa.
(This roughly corresponds to equality on trees modulo alpha renaming and reordering of alternatives.)
The rewrite only applies if some of the cases are guarded (this must be checked before invoking this method).
The rewrite goes through the switch top-down and merges each case with the subsequent cases it is implied by
(i.e. it matches if they match, not taking guards into account)
If there are no unreachable cases, all cases can be uniquely assigned to a partition of such 'overlapping' cases,
save for the default case (thus we jump to it rather than copying it several times).
(The cases in a partition are implied by the principal element of the partition.)
The overlapping cases are merged into one case with their guards pushed into the body as follows
(with P the principal element of the overlapping patterns Pi):
`{case Pi if(G_i) => B_i }*` is rewritten to `case P => {if(G_i) B_i}*`
The rewrite fails (and returns Nil) when:
(1) there is a subsequence of overlapping cases that has an unguarded case in the middle;
only the last case of each subsequence of overlapping cases may be unguarded (this is implied by unreachability)
(2) there are overlapping cases that differ (tested by `caseImpliedBy`)
cases with patterns A and B are overlapping if for SOME value x, A matches x implies B matches y OR vice versa <-- note the difference with case equality defined above
for example `case 'a' | 'b' =>` and `case 'b' =>` are different and overlapping (overlapping and equality disregard guards)
Improved by @retronym's feedback in the following ways:
- fix patternEquals (it's now quadratic, but correct)
- update neg/t6011 to test the improved patternEquals
- remove side-effect-in-condition ugliness
- introduce isGuardedCase
- docs & various code clarity
Also closes SI-6048 (duplicate).
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